1. Field of the Invention
The present invention relates to a control circuit, and more particularly, to a power voltage control circuit for a central processing unit (CPU).
2. Description of the Related Art
CPUs in personal computers generally have at least one power indication (VID) pin to output a power indication signal SVID and to control the received power voltage, as shown in FIG. 1. When CPUs of different manufacture require different power voltages, the desired power voltage can be determined according to the power indication signal from the PID pin.
In the conventional method a CPU is overclocked by adjusting both the operating frequency of the CPU and the power voltage of the CPU. As shown in FIG. 2, a default value can be stored in the VID register 16 through an interface bus 14 such as a system bus, an I2C bus, or the like. VID controller 18 produces a selection signal Sel, and multiplexer (MUX) 20 outputs the signal SVID from the CPU 10 or the signal RVID from the VID register 16. The MUX 20 outputs the signal RVID to PWM controller 12 according to the selection signal Sel when the CPU is overclocked, so that the correct power voltage is output to the CPU.
The signal SVID from the power indication pin, however, fluctuates continuously as the load and operating temperature of the CPU 10 (based on specification VRD 10.0 proposed by Intel) fluctuates. As shown in FIG. 2, the control signal SVID from the CPU is isolated from the PWM controller 12 by the MUX 20 according to selection signal Sel. Consequently, the control circuit cannot adjust the power voltage output to the CPU 10 as CPU load and operating temperature fluctuate. Thus, the conventional control circuit as shown in FIG. 2 cannot efficiently provide the correct voltage necessary to overclock the CPU 10.